The invention herein relates to stress sensors and, in particular, to semiconductor material stress sensors based on the piezoresistive effect.
The use of the piezoresistive effect in silicon as the transduction basis for a stress sensor is well known and such stress sensors are widely used. Typically, a p-type conductivity region is formed in an n-type conductivity silicon layer by diffusion so that the p-type region serves as pn junction isolated resistor. Usually, this resistor is provided in a diaphragm structure over which the stress to be measured is exerted. By electrically contacting the p-type region or resistor at two points and measuring the resistance change between these contacts during applications of stress due to the piezoresistive effect, the magnitude of the applied stress can be determined.
To protect such surface diffused p-type conductivity regions serving as resistors for sensing stress through the piezoresistive effect, i.e. piezoresistors, from contaminants, etc., they and the immediate surroundings must be covered by some protective material. Usually, a silicon dioxide layer is provided on the surface of the silicon to cover at least the junction area between the piezoresistor and its surroundings and, most often, the entire silicon surface.
The silicon dioxide-silicon interface occurs, of course, at the silicon surface diffused from which is approximately where the dopant concentration of the p-type conductivity diffused region, or piezoresistor, is a maximum, a characteristic of the surface diffusion process. Silicon dioxide will, during its formation, take up some kinds of p-type diffusants, or dopants, and therefore, the maximum dopant concentration occurring at the silicon surface can be substantially altered from the initially predeposited maximum concentration. This will reduce the maximum dopant concentration in the silicon as well as the total amount of dopant in the silicon after diffusion. The maximum dopant concentration value has been found to be strongly related to the stress sensor transduction performance over temperature, that performance being the resistance change versus applied stress as a function of temperature over a range of temperatures, that is, the piezoresistive temperature coefficient. The total resistance value of the piezoresistor at any given temperature is, of course, related to the total amount of dopant provided in the p-type conductivity region.
Thus, both the resistance value of the piezoresistor at a particular temperature and the resistance values of the piezoresistor versus stress over a temperature range are subject to substantial changes because of the formation of a protective silicon dioxide layer. Because the effects of forming such a silicon dioxide layer vary substantially from sensor unit to sensor unit, particularly when the sensor units are from distinct process runs, the piezoresistors resulting differ from the piezoresistor intended in an unpredictable manner, i.e. the resulting piezoresistors are nonuniform or, in other words, have characteristics differing considerably from sensor unit to sensor unit. This variability or nonuniformity presents difficult problems in providing compensation schemes to produce a stress sensing system output signal which will accurately reflect the external stress exerted upon the stress sensor.
Piezoresistor nonuniformity is not the only cause of inaccurate stress sensor performance in the above-described stress sensor construction. The silicon dioxide itself and any metallic interconnection leads to or across the piezoresistor or any other materials occurring over the diaphragm will alter the diaphragm thermal and mechanical response to changing temperatures and to applied stress from that which it would be without these constraints present on the diaphragm surface. This alteration of diaphragm responses, leading to further errors in determining the external stresses exerted on the diaphragm, occur because of the internally generated, temperature dependent stresses and the thermal and mechanical hysteresis at the interfaces of the diaphragm and its covering materials. The extent of these effects will vary considerably from stress sensor unit to stress sensor unit.
Internally generated stresses occur because of mismatches in the thermal expansion characteristics of two or more materials present in the diaphragm. Hysteresis occurs when two or more of the materials slip with respect to one another as the result of stress or temperature change.
A typical situation in which these internally generated stresses detrimentally occur is that in which the use is made of a silicon nitride layer over the silicon dioxide or use is made of an organic material over the silicon dioxide to prevent contamination of the silicon dioxide by ions. The use of an organic material will typically also lead to difficult hysteresis problems.
Any attempt to cure hysteresis problems in a relatively low cost sensing system can only make economic sense by being directed toward minimizing the problem rather than being directed to compensating the hysteresis effects. This is because compensation requires a memory system to remember the history of the hysteresis source over the last cycle of the hysteresis loop. The problem is compounded by the variability in hysteresis loops from sensor to sensor. Thus, adding significantly to hysteresis problem is to be strongly avoided. Increasing the variability temperature coefficient because of temperature dependent, internally generated stresses is also clearly undesirable because of the difficulties introduced in providing a temperature compensation scheme.
However, use of such additional covering materials to prevent ion contamination of the silicon dioxide covering layer is often attempted because of substantial changes which can occur in piezoresistor characteristics over time resulting from such ion contamination. Due to (i) mobile ions which are contaminants in the silicon dioxide and which can reach the silicon dioxide-silicon interface, and (ii) fixed ionic charges also contaminating the silicon dioxide which are able to induce charges at that silicon dioxide-silicon interface, the diffused resistors which are to be protected are also rendered electrically unstable over time to a greater or lesser degree by the silicon dioxide layer provided.
This time instability results because the charges appearing at the silicon dioxide-silicon interface convert portions of the silicon layer to a p-type conductivity even though these portions of the silicon layer were in the initially intended n-type conductivity silicon regions and outside the initially intended p-type conductivity region. These additions to the intended p-type region enlarge that region, and so reduce the resistance value intended for the diffused p-type region, and, further, these additions add area to the pn junction, i.e. semiconductor junction, which occurs between adjacent regions of opposite conductivity types. The added p-type region portions tend to reduce the resistance initially intended to occur in the p-type region by enlargement thereof, as already mentioned, and to also reduce intended resistance by the increasing of the semiconductor junction area. The increased junction area leads to increases in the junction leakage current which has the effect of reducing the intended resistance.
The charge occurring at the silicon dioxide-silicon interface in the above-described sensor construction is a source of electrical instability over time because the charge changes in both amount and location thereby altering the p-type conductivity region, or piezoresistor formed, in the silicon layer. These interface charges are highly variable in amount present initially and over time in depending strongly on operating environment in use and on the details of the processing in any given sensor fabrication sequence. The mobile charges are also vary in location in the silicon dioxide, and so in effect over time, depending on the voltages and temperatures experienced in use.
Signals indicating applied external stress provided by the piezoresistors in semiconductor material stress sensors are almost always supplied to signal processing circuitry for modification. At the very least, stress sensor output in response to applied stress will always shift with temperature for a silicon piezoresistor, i.e. there is a temperature coefficient associated with every piezoresistor, as in indicated in FIG. 1. There, resistance change, .DELTA.R, versus stress, S, is plotted for three different absolute temperatures, T.sub.1 to T.sub.3, and for two different maximum dopant concentrations, MC.sub.1 and MC.sub.2. The total number of acceptor atoms, Np, is held constant.
This temperature performance of the piezoresistors must always be compensated by signal processing circuitry if the sensing system is to provide an accurate indication of the external stress applied thereto. If the characteristics shown in FIG. 1 are (i) uniform from unit to unit, (ii) constant over time, and (iii) independent of the history of applied heat and stress, then signal processing circuitry can be designed to just cancel out the temperature dependence of the piezoresistors so that output signals from the signal processing are always an accurate indication of the stress exerted on the stress sensor.
However, as discussed in the preceding, the addition of a silicon dioxide layer over the silicon surface containing surface diffused piezoresistors leads to nonuniform piezoresistors, to degraded thermal and mechanical responses of piezoresistors and to instability over time of the piezoresistor characteristics. In many instances, it is entirely impractical to attempt to compensate for hysteresis at all. If the temperature performance is not the same from sensor unit to sensor unit because of nonuniformities in the piezoresistor construction, then the compensation scheme must be adjusted for each unit. Similar provisions must be made for nonuniform resistance values of the piezoresistors from unit to unit. Such adjustments are a time consuming and expensive operation. And, if the temperature performance and resistance values change over time in use, the compensation scheme must also be adjusted over time, an often prohibitive requirement for many potential uses.